Dating Methods

World of Earth Science
COPYRIGHT 2003 The Gale Group, Inc.

Dating methods

Dating techniques are procedures used by scientists to determine the age of a specimen. Relative dating methods tell only if one sample is older or younger than another sample; absolute dating methods provide a date in years. The latter have generally been available only since 1947. Many absolute dating techniques take advantage of radioactive decay, whereby a radioactive form of an element is converted into another radioactive isotope or non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have undergone continual refinement as scientists strive to develop the most accurate dating techniques possible.

Relative dating methods determine whether one sample is older or younger than another. They do not provide an age in years. Before the advent of absolute dating methods, nearly all dating was relative. The main relative dating method is stratigraphy .

Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which, except at unconformities , nearly always holds true) that deeper layers were deposited earlier, and thus are older than more shallow layers. The sequential layers of rock represent sequential intervals of time. Although these units may be sequential, they are not necessarily continuous due to erosional removal of some intervening units. The smallest of these rock units that can be matched to a specific time interval is called a bed. Beds that are related are grouped together into members, and members are grouped into formations.

Seriation is the ordering of objects according to their age. It is a relative dating method. In a landmark study, archaeologist James Ford used seriation to determine the chronological order of American Indian pottery styles in the Mississippi Valley. Artifact styles such as pottery types are seriated by analyzing their abundances through time. This is done by counting the number of pieces of each style of the artifact in each stratigraphic layer and then graphing the data. A layer with many pieces of a particular style will be represented by a wide band on the graph, and a layer with only a few pieces will be represented by a narrow band. The bands are arranged into battleship-shaped curves, with each style getting its own curve. The curves are then compared with one another, and from this the relative ages of the styles are determined. A limitation to this method is that it assumes all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study.

The term faunal dating refers to the use of animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Scientists can determine an approximate age for a layer by examining which species or genera of animals are buried in it. The technique works best if the animals belonged to species that evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. In addition to providing rough absolute dates for specimens buried in the same stratigraphic unit as the bones, faunal analysis can also provide relative ages for objects buried above or below the fauna-encasing layers.

Each year seed-bearing plants release large numbers of pollen grains. This process results in a "rain" of pollen that falls over many types of environments. Pollen that ends up in lakebeds or peat bogs is the most likely to be preserved, but pollen may also become fossilized in arid conditions if the soil is acidic or cool. Scientists can develop a pollen chronology, or calendar, by noting which species of pollen were deposited earlier in time, that is, residue in deeper sediment or rock layers, than others. A pollen zone is a period of time in which a particular species is much more abundant than any other species of the time. In most cases, this also reveals much about the climate of the period, because most plants only thrive in specific climatic conditions. Changes in pollen zones can also indicate changes in human activities such as massive deforestation or new types of farming. Pastures for grazing livestock are distinguishable from fields of grain, so changes in the use of the land over time are recorded in the pollen history. The dates when areas of North America were first settled by immigrants can be determined to within a few years by looking for the introduction of ragweed pollen.

Pollen zones are translated into absolute dates by the use of radiocarbon dating. In addition, pollen dating provides relative dates beyond the limits of radiocarbon (40,000 years), and can be used in some places where radiocarbon dates are unobtainable.

Fluorine is found naturally in ground water . This water comes in contact with skeletal remains under ground. When this occurs, the fluorine in the water saturates the bone, changing the mineral composition. Over time, more and more fluorine incorporates itself into the bone. By comparing the relative amounts of fluorine composition of skeletal remains, one can determine whether the remains were buried at the same time. A bone with a higher fluorine composition has been buried for a longer period of time.

Absolute dating is the term used to describe any dating technique that tells how old a specimen is in years. These are generally analytical methods, and are carried out in a laboratory. Absolute dates are also relative dates, in that they tell which specimens are older or younger than others. Absolute dates must agree with dates from other relative methods in order to be valid.

This dating technique of amino acid racimization was first conducted by Hare and Mitterer in 1967, and was popular in the 1970s. It requires a much smaller sample than radiocarbon dating, and has a longer range, extending up to a few hundred thousand years. It has been used to date coprolites (fossilized feces) as well as fossil bones and shells. These types of specimens contain proteins embedded in a network of minerals such as calcium.

Amino acid racimization is based on the principle that amino acids (except glycine, a very simple amino acid) exist in two mirror image forms called stereoisomers. Living organisms (with the exception of some microbes) synthesize and incorporate only the L-form into proteins. This means that the ratio of the D-form to the L-form is zero (D/L=0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process called racimization. This occurs because protons (H+) are removed from the amino acids by acids or bases present in the burial environment. The protons are quickly replaced, but will return to either side of the amino acid, not necessarily to the side from which they came. This may form a D-amino acid instead of an L–amino acid. The reversible reaction eventually creates equal amounts of L–and D-forms (D/L=1.0).

The rate at which the reaction occurs is different for each amino acid; in addition, it depends upon the moisture, temperature , and pH of the postmortem conditions. The higher the temperature, the faster the reaction occurs, so the cooler the burial environment, the greater the dating range. The burial conditions are not always known, however, and can be difficult to estimate. For this reason, and because some of the amino acid racimization dates have disagreed with dates achieved by other methods, the technique is no longer widely used.

Cation-ratio dating is used to date rock surfaces such as stone artifacts and cliff and ground drawings. It can be used to obtain dates that would be unobtainable by more conventional methods such as radiocarbon dating. Scientists use cation-ratio dating to determine how long rock surfaces have been exposed. They do this by chemically analyzing the varnish that forms on these surfaces. The varnish contains cations, which are positively charged atoms or molecules. Different cations move throughout the environment at different rates, so the ratio of different cations to each other changes over time. Cation ratio dating relies on the principle that the cation ratio (K++Ca2+)/Ti4+ decreases with increasing age of a sample. By calibrating these ratios with dates obtained from rocks from a similar microenvironment, a minimum age for the varnish can be determined. This technique can only be applied to rocks from desert areas, where the varnish is most stable.

Although cation-ratio dating has been widely used, recent studies suggest it has potential errors. Many of the dates obtained with this method are inaccurate due to improper chemical analyses. In addition, the varnish may not actually be stable over long periods of time.

Thermoluminescence dating is very useful for determining the age of pottery. Electrons from quartz and other minerals in the pottery clay are bumped out of their normal positions (ground state) when the clay is exposed to radiation. This radiation may come from radioactive substances such as uranium,

present in the clay or burial medium, or from cosmic radiation. When the ceramic is heated to a very high temperature (over 932°F [500°C]), these electrons fall back to the ground state, emitting light in the process and resetting the "clock" to zero. The longer the radiation exposure, the more electrons get bumped into an excited state. With more electrons in an excited state, more light is emitted upon heating. The process of displacing electrons begins again after the object cools. Scientists can determine how many years have passed since a ceramic was fired by heating it in the laboratory and measuring how much light is given off. Thermoluminescence dating has the advantage of covering the time interval between radiocarbon and potassium-argon dating, or 40,000–200,000 years. In addition, it can be used to date materials that cannot be dated with these other two methods.

Optically stimulated luminescence (OSL) has only been used since 1984. It is very similar to thermoluminescence dating, both of which are considered "clock setting" techniques. Minerals found in sediments are sensitive to light. Electrons found in the sediment grains leave the ground state when exposed to light, called recombination. To determine the age of sediment, scientists expose grains to a known amount of light and compare these grains with the unknown sediment. This technique can be used to determine the age of unheated sediments less than 500,000 years old. A disadvantage to this technique is that in order to get accurate results, the sediment to be tested cannot be exposed to light (which would reset the "clock"), making sampling difficult.

The absolute dating method utilizing tree ring growth is known as dendrochronology. It is based on the fact that trees produce one growth ring each year. Narrow rings grow in cold and/or dry years, and wide rings grow in warm years with plenty of moisture. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area. The patterns from trees of different ages (including ancient wood) are overlapped, forming a master pattern that can be used to date timbers thousands of years old with a resolution of one year. Timbers can be used to date buildings and archaeological sites. In addition, tree rings are used to date changes in the climate such as sudden cool or dry periods. Dendrochronology has a range of one to 10,000 years or more.

As previously mentioned, radioactive decay refers to the process in which a radioactive form of an element is converted into a decay product at a regular rate. Radioactive decay dating is not a single method of absolute dating but instead a group of related methods for absolute dating of samples.

Potassium-argon dating relies on the fact that when volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon-40 (40Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (40K). The amount of 40Ar formed is proportional to the decay rate (half-life ) of 40K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the 40K originally present to be converted into 40Ar. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated. The reason such old material is required is that it takes a very long time to accumulate enough 40Ar to be measured accurately. Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa .

Radiocarbon dating is used to date charcoal, wood, and other biological materials. The range of conventional radiocarbon dating is 30,000–40,000 years, but with sensitive instrumentation, this range can be extended to 70,000 years. Radiocarbon (14C) is a radioactive form of the element carbon . It decays spontaneously into nitrogen-14 (14N). Plants get most of their carbon from the air in the form of carbon dioxide , and animals get most of their carbon from plants (or from animals that eat plants). Relative to their atmospheric proportions, atoms of 14C and of a non-radioactive form of carbon, 12C, are equally likely to be incorporated into living organisms. While a plant or animal is alive, the ratio of 14C/12C in its body will be nearly the same as the 14C/12C ratio in the atmosphere. When the organism dies, however, its body stops incorporating new carbon. The ratio will then begin to change as the 14C in the dead organism decays into 14N. The rate at which this process occurs is called the half-life. This is the time required for half of the 14C to decay into 14N. The half-life of 14C is 5,730 years. Scientists can estimate how many years have elapsed since an organism died by comparing the 14C/12C ratio in the remains with the ratio in the atmosphere. This allows them to determine how much 14C has formed since the death of the organism.

One of the most familiar applications of radioactive dating is determining the age of fossilized remains, such as dinosaur bones. Radioactive dating is also used to authenticate the age of rare archaeological artifacts. Because items such as paper documents and cotton garments are produced from plants, they can be dated using radiocarbon dating. Without radioactive dating, a clever forgery might be indistinguishable from a real artifact. There are some limitations, however, to the use of this technique. Samples that were heated or irradiated at some time may yield by radioactive dating an age less than the true age of the object. Because of this limitation, other dating techniques are often used along with radioactive dating to ensure accuracy.

Accurate radiocarbon dating is that diagenic (after death) demands consideration regarding potential contamination of a specimen and a proper application of changes in the 14C/12C ratio in the atmosphere over time. 14C levels can be measured in tree rings and used to correct for the 14C/12C ratio in the atmosphere at the time the organism died, and can even be used to calibrate some dates directly. Although the magnitude of change of the 14C/12C ratio sometimes stirs controversy, with proper calibration and correction, radiocarbon dating correlates well with other dating techniques and consistently proves to be an accurate dating technique—especially for Pleistocene and Holocene period analysis.

Uranium series dating techniques rely on the fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive "daughter" isotopes; this process continues until a stable (non-radioactive) lead isotope is formed. The daughters have relatively short half-lives ranging from a few hundred thousand years down to only a few years. The "parent" isotopes have half-lives of several billion years. This provides a dating range for the different uranium series of a few thousand years to 500,000 years. Uranium series have been used to date uranium-rich rocks, deep-sea sediments, shells, bones, and teeth, and to calculate the ages of ancient lakebeds. The two types of uranium series dating techniques are daughter deficiency methods and daughter excess methods.

In daughter deficiency situations, the parent radioisotope is initially deposited by itself, without its daughter (the isotope into which it decays) present. Through time, the parent decays to the daughter until the two are in equilibrium (equal amounts of each). The age of the deposit may be determined by measuring how much of the daughter has formed, providing that neither isotope has entered or exited the deposit after its initial formation. Carbonates may be dated this way using, for example, the daughter/parent isotope pair protactinium-231/uranium-235 (231Pa/235U). Living mollusks and corals will only take up dissolved compounds such as isotopes of uranium, so they will contain no protactinium, which is insoluble. Protactinium-231 begins to accumulate via the decay of 235U after the organism dies. Scientists can determine the age of the sample by measuring how much 231Pa is present and calculating how long it would have taken that amount to form.

In the case of daughter excess, a larger amount of the daughter is initially deposited than the parent. Non-uranium daughters such as protactinium and thorium are insoluble, and precipitate out on the bottoms of bodies of water, forming daughter excesses in these sediments. Over time, the excess daughter disappears as it is converted back into the parent, and by measuring the extent to which this has occurred, scientists can date the sample. If the radioactive daughter is an isotope of uranium, it will dissolve in water, but to a different extent than the parent; the two are said to have different solubilities. For example, 234U dissolves more readily in water than its parent, 238U, so lakes and oceans contain an excess of this daughter isotope. This excess is transferred to organisms such as mollusks or corals, and is the basis of 234U/238U dating.

Some volcanic minerals and glasses, such as obsidian , contain uranium-238 (238U). Over time, these substances become "scratched." The marks, called tracks, are the damage caused by the fission (splitting) of the uranium atoms. When an atom of 238U splits, two "daughter" atoms rocket away from each other, leaving in their wake tracks in the material in which they are embedded. The rate at which this process occurs is proportional to the decay rate of 238U. The decay rate is measured in terms of the half-life of the element, or the time it takes for half of the element to split into its daughter atoms. The half-life of 238U is 4.47x109 years.

When the mineral or glass is heated, the tracks are erased in much the same way cut marks fade away from hard candy that is heated. This process sets the fission track clock to zero, and the number of tracks that then form are a measure of the amount of time that has passed since the heating event. Scientists are able to count the tracks in the sample with the aid of a powerful microscope. The sample must contain enough 238U to create enough tracks to be counted, but not contain too much of the isotope, or there will be a jumble of tracks that cannot be distinguished for counting. One of the advantages of fission track dating is that it has an enormous dating range. Objects heated only a few decades ago may be dated if they contain relatively high levels of 238U; conversely, some meteorites have been dated to over a billion years old with this method.

Although certain dating techniques are accurate only within certain age ranges, whenever possible, scientists attempt to use multiple methods to date specimens. Correlation of dates via different dating methods provides a highest degree of confidence in dating.

Dating Techniques

Mathematics
COPYRIGHT 2002 The Gale Group Inc.

Dating Techniques

Movies and television have presented a romantic vision of archaeology as adventure in far-away and exotic locations. A more realistic picture might show researchers digging in smelly mud for hours under the hot sun while battling relentless mosquitoes. This type of archaeological research produces hundreds of small plastic bags containing pottery shards, animal bones, bits of worked stone, and other fragments. These findings must be classified, which requires more hours of tedious work in a stuffy tent. At its best, archaeology involves a studious examination of the past with the goal of learning important information about the culture and customs of ancient (or not so ancient) peoples. Much archaeology in the early twenty-first century investigates the recent past, a sub-branch called "historical archaeology."

What Is Archaeology?

Archaeology is the study of the material remains of past human cultures. It is distinguished from other forms of inquiry by its method of study, excavation. (Most archaeologists call this "digging.") Excavation is not simply digging until something interesting is found. That sort of unscientific digging destroys the archaeological information. Archaeological excavation requires the removal of material layer by layer to expose artifacts in place. The removed material is carefully sifted to find small artifacts , tiny animal bones, and other remains. Archaeologists even examine the soil in various layers for microscopic material, such as pollen. Excavations, in combination with surveys, may yield maps of a ruin or collections of artifacts.

Time is important to archaeologists. There is rarely enough time to complete the work, but of even greater interest is the time that has passed since the artifact was created. An important part of archaeology is the examination of how cultures change over time. It is therefore essential that the archaeologist is able to establish the age of the artifacts or other material remains and arrange them in a chronological sequence. The archaeologist must be able to distinguish between objects that were made at the same time and objects that were made at different times. When objects that were made at different times are excavated, the archaeologist must be able to arrange them in a sequence from the oldest to the most recent.

Relative Dating and Absolute Dating

Before scientific dating techniques such as dendrochronology and radiocarbon dating were introduced to archaeology, the discipline was dominated by extensive discussions of the chronological sequence of events. Most of those questions have now been settled and archaeologists have moved on to other issues. Scientific dating techniques have had a huge impact on archaeology.

Archaeologists use many different techniques to determine the age of an object. Usually, several different techniques are applied to the same object. Relative dating arranges artifacts in a chronological sequence from oldest to most recent without reference to the actual date. For example, by studying the decorations used on pottery, the types of materials used in the pottery, and the types and shapes of pots, it is often possible to arrange them into a sequence without knowing the actual date. In absolute dating , the age of an object is determined by some chemical or physical process without reference to a chronology.

Relative Dating Methods. The most common and widely used relative dating technique is stratigraphy. The principle of superposition (borrowed from geology) states that higher layers must be deposited on top of lower layers. Thus, higher layers are more recent than lower layers. This only applies to undisturbed deposits. Rodent burrows, root action, and human activity can mix layers in a process known as bioturbation. However, the archaeologist can detect bioturbation and allow for its effects.

Discrete layers of occupation can often be determined. For example, Hisarlik, which is a hill in Turkey, is thought by some archaeologists to be the site of the ancient city of Troy. However, Hisarlik was occupied by many different cultures at various times both before and after the time of Troy, and each culture built on top of the ruins of the previous culture, often after violent conquest. Consequently, the layers in this famous archaeological site represent many different cultures. An early excavator of Hisarlik, Heinrich Schleimann, inadvertently dug through the Troy layer into an earlier occupation and mistakenly assigned the gold artifacts he found there to Troy. Other sites have been continuously occupied by the same culture for a long time and the different layers represent gradual changes. In both cases, stratigraphy will apply.

A chronology based on stratigraphy often can be correlated to layers in other nearby sites. For example, a particular type or pattern of pottery may occur in only one layer in an excavation. If the same pottery type is found in another excavation nearby, it is safe to assume that the layers are the same age. Archaeologists rarely make these determinations on the basis of a single example. Usually, a set of related artifacts is used to determine the age of a layer.

Seriation simply means ordering. This technique was developed by the inventor of modern archaeology, Sir William Matthew Flinders Petrie. Seriation is based on the assumption that cultural characteristics change over time. For example, consider how automobiles have changed in the last 50 years (a relatively short time in archaeology). Automobile manufacturers frequently introduce new styles about every year, so archaeologists thousands of years from now will have no difficulty identifying the precise date of a layer if the layer contains automobile parts.

Cultural characteristics tend to show a particular pattern over time. The characteristic is introduced into the culture (for example, using a certain type of projectile point for hunting or wearing low-riding jeans), becomes progressively more popular, then gradually wanes in popularity. The method of seriation uses this distinctive pattern to arrange archaeological materials into a sequence. However, seriation only works when variations in a cultural characteristic are due to rapid and significant change over time. It also works best when a characteristic is widely shared among many different members of a group. Even then, it can only be applied to a small geographic area, because there is also geographic variation in cultural characteristics. For example, 50 years ago American automobiles changed every year while the Volkswagen Beetle hardly changed at all from year to year.

Cross dating is also based on stratigraphy. It uses the principle that different archaeological sites will show a similar collection of artifacts in layers of the same age. Sir Flinders Petrie used this method to establish the time sequence of artifacts in Egyptian cemeteries by identifying which burials contained Greek pottery vessels. These same Greek pottery styles could be associated with monuments in Greece whose construction dates were fairly well known. Since absolute dating techniques have become common, the use of cross dating has decreased significantly.

Pollen grains also appear in archaeological layers. They are abundant and they survive very well in archaeological contexts. As climates change over time, the plants that grow in a region change as well. People who examine pollen grains (the study of which is known as pollen analysis ) can usually determine the genus , and often the exact species producing a certain pollen type. Archaeologists can then use this information to determine the relative ages of some sites and layers within sites. However, climates do not change rapidly, so this type of analysis is best for archaeological sites dating back to the last ice age.

Absolute Dating Methods. Absolute dating methods produce an actual date, usually accurate to within a few years. This date is established independent of stratigraphy and chronology. If a date for a certain layer in an excavation can be established using an absolute dating method, other artifacts in the same layer can safely be assigned the same age.

Dendrochronology, also known as tree-ring dating, is the earliest form of absolute dating. This method was first developed by the American astronomer Andrew Ellicott Douglas at the University of Arizona in the early 1900s. Douglas was trying to develop a correlation between climate variations and sunspot activity , but archaeologists quickly recognized its usefulness as a dating tool. The technique was first applied in the American Southwest and later extended to other parts of the world.

Tree-ring dating is relatively simple. Trees add a new layer of cambium (the layer right under the bark) every year. The thickness of the layer depends on local weather and climate. In years with plenty of rain, the layer will be thick and healthy. Over the lifetime of the tree, these rings accumulate, and the rings form a record of regional variation in climate that may extend back hundreds of years. Since all of the trees in a region experience the same climate variations, they will have similar growth patterns and similar tree ring patterns.

One tree usually does not cover a period sufficiently long to be archaeologically useful. However, patterns of tree ring growth have been built up by "overlapping" ring sequences from different trees so that the tree ring record extends back several thousand years in many parts of the world. The process starts with examination of the growth ring patterns of samples from living trees. Then older trees are added to the sequence by overlapping the inner rings of a younger sample with the outer rings of an older sample. Older trees are recovered from old buildings, archaeological sites, peat bogs, and swamps. Eventually, a regional master chronology is constructed.

When dendrochronology can be used, it provides the most accurate dates of any technique. In the American Southwest, the accuracy and precision of dendrochronology has enabled the development of one of the most

accurate prehistoric cultural chronologies anywhere in the world. Often events can be dated to within a decade. This precision has allowed archaeologists working in the American Southwest to reconstruct patterns of village growth and subsequent abandonment with a fineness of detail unmatched in most of the world.

Radiometric dating methods are more recent than dendrochronology. However, dendrochronology provides an important calibration technique for radiocarbon dating techniques. All radiometric-dating techniques are based on the well-established principle from physics that large samples of radioactive isotopes decay at precisely known rates. The rate of decay of a radioactive isotope is usually given by its half-life. The decay of any individual nucleus is completely random. The half-life is a measure of the probability that a given atom will decay in a certain time. The shorter the half-life, the more likely the atom will decay. This probability does not increase with time. If an atom has not decayed, the probability that it will decay in the future remains exactly the same. This means that no matter how many atoms are in a sample, approximately one-half will decay in one half-life. The remaining atoms have exactly the same decay probability, so in another half-life, one half of the remaining atoms will decay. The amount of time required for one-half of a radioactive sample to decay can be precisely determined. The particular radioisotope used to determine the age of an object depends on the type of object and its age.

Radiocarbon is the most common and best known of radiometric dating techniques, but it is also possibly the most misunderstood. It was developed at the University of Chicago in 1949 by a group of American scientists led by Willard F. Libby. Radiocarbon dating has had an enormous impact on archaeology. In the last 50 years, radiocarbon dating has provided the basis for a worldwide cultural chronology. Recognizing the importance of this technique, the Nobel Prize committee awarded the Prize in Chemistry to Libby in 1960.

The physics behind radiocarbon dating is straightforward. Earth's atmosphere is constantly bombarded with cosmic rays from outer space. Cosmic-ray neutrons collide with atoms of nitrogen in the upper atmosphere, converting them to atoms of radioactive carbon-14. The carbon-14 atom quickly combines with an oxygen molecule to form carbon dioxide. This radioactive carbon dioxide spreads throughout Earth's atmosphere, where it is taken up by plants along with normal carbon-12. As long as the plant is alive, the relative amount (ratio) of carbon-14 to carbon-12 remains constant at about one carbon-14 atom for every one trillion carbon-12 atoms. Some animals eat plants and other animals eat the plant-eaters. As long as they are alive, all living organisms have the same ratio of carbon-14 to carbon-12 as in the atmosphere because the radioactive carbon is continually replenished, either through photosynthesis or through the food animals eat.

However, when the plant or animal dies, the intake of carbon-14 stops and the ratio of carbon-14 to carbon-12 immediately starts to decrease. The half-life of carbon-14 is 5,730 years. After 5,730 years, about one-half of the carbon-14 atoms will have decayed. After another 5,730 years, one-half of the remaining atoms will have decayed. So after 11,460 years, only one-fourth will remain. After 17,190 years, one-eighth of the original carbon-14 will remain. After 22,920 years, one-sixteenth will remain.

Radiocarbon dating has become the standard technique for determining the age of organic remains (those remains that contain carbon). There are many factors that must be taken into account when determining the age of an object. The best objects are bits of charcoal that have been preserved in completely dry environments. The worst candidates are bits of wood that have been saturated with sea water, since sea water contains dissolved atmospheric carbon dioxide that may throw off the results. Radiocarbon dating can be used for small bits of clothing or other fabric, bits of bone, baskets, or anything that contains organic material.

There are well over 100 labs worldwide that do radiocarbon dating. In the early twenty-first century, the dating of objects up to about 10 half-lives, or up to about 50,000 years old, is possible. However, objects less than 300 years old cannot be reliably dated because of the widespread burning of fossil fuels, which began in the nineteenth century, and the production of carbon-14 from atmospheric testing of nuclear weapons in the 1950s and 1960s. Another problem with radiocarbon dating is that the production of carbon-14 in the atmosphere has not been constant, due to variation in solar activity. For example, in the 1700s, solar activity dropped (a phenomenon called the "Maunder Minimum"), so carbon-14 production also decreased during this period. To achieve the highest level of accuracy, carbon-14 dates must be calibrated by comparison to dates obtained from dendrochronology.

Calibration of Radiocarbon Dates. Samples of Bristlecone pine, a tree with a very long life span, have been dated using both dendrochronology and radiocarbon dating. The results do not agree, but the differences are consistent. That is, the radiocarbon dates were always wrong by the same number of years. Consequently, tree-ring chronologies have been used to calibrate radiocarbon dates to around 12,000 years ago.

When radiocarbon dating was first put into use, it was decided that dates would always be reported as B.P., where B.P. stood for "before present" and "present" was defined as 1950. That way, dates reported in magazine articles and books do not have to be adjusted as the years pass. So if a lab determines that an object has a radiocarbon age of 1,050 years in 2000, its age will be given as 1000 B.P. Calibrated dates are given using the actual date, such as 950 c.e.

Potassium-Argon Dating. If an object is too old to be dated by radiocarbon dating, or if it contains no organic material, other methods must be used. One of these is potassium-argon dating. All naturally occurring rocks contain potassium. Some of the potassium in rocks is the radioactive isotope potassium-40. Potassium-40 gradually decays to the stable isotope argon-40, which is a gas. When the rock is melted, as in a volcano, any argon gas trapped in the rock escapes. When the rock cools, the argon will begin to build up. So this method can be used to measure the age of any volcanic rock, from 100,000 years up to around 5 billion years old.

This method is not widely used in archaeology, since most archaeological deposits are not associated with volcanic activity. However, Louis and Mary Leakey successfully used the method to determine the ages of fossils in Olduvai Gorge in Tanzania by examining rocks from lava flows above and below the fossils. They were able to establish an absolute chronology for humans and human ancestors extending back two million years. At Laetolli, in Tanzania, volcanic ash containing early hominid footprints was dated by this method at 3.5 million years.

Other Methods. Uranium-238 is present in most rocks. This isotope of uranium spontaneously undergoes fission . The fission fragments have a lot of energy, and they plow through the rock, leaving a track that can be made visible by treating the rock. So by counting fission tracks, the age of the rock can be determined. Like potassium-argon dating, this can only be used to determine the age of the rock, not the age of the artifact itself.

Thermoluminescence is a recently developed technique that uses the property of some crystals to "store" light. Sometimes an electron will be knocked out of its position in a crystal and will "stick" somewhere else in the crystal. These displaced electrons will accumulate over time. If the sample is heated, the electrons will fall back to their normal positions, emitting a small flash of light. By measuring the light emitted, the time that has passed since the artifact was heated can be determined. This method should prove to be especially useful in determining the age of ceramics, rocks that have been used to build fire rings, and samples of chert and flint that have been deliberately heated to make them easier to flake into a projectile point.

Conclusion

Science continues to develop new methods to determine the age of objects. As our knowledge of past chronologies improves, archaeologists will be better able to understand how cultures change over time, and how different cultures interact with each other. As a result, this knowledge will enable us to achieve a progressively better understanding of our own culture.

Dating Techniques

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COPYRIGHT 2002 The Gale Group, Inc.

Dating techniques

Dating techniques are procedures used by scientists to determine the age of an object or a series of events. The two main types of dating methods are relative and absolute. Relative dating methods are used to determine only if one sample is older or younger than another. Absolute dating methods are used to determine an actual date in years for the age of an object.

Relative dating

Before the advent of absolute dating methods in the twentieth century, nearly all dating was relative. The main relative dating method is stratigraphy (pronounced stra-TI-gra-fee), which is the study of layers of rocks or the objects embedded within those layers. This method is based on the assumption (which nearly always holds true) that deeper layers of rock were deposited earlier in Earth's history, and thus are older than more shallow layers. The successive layers of rock represent successive intervals of time.

Since certain species of animals existed on Earth at specific times in history, the fossils or remains of such animals embedded within those successive layers of rock also help scientists determine the age of the layers. Similarly, pollen grains released by seed-bearing plants became fossilized in rock layers. If a certain kind of pollen is found in an archaeological site, scientists can check when the plant that produced that pollen lived to determine the relative age of the site.

Absolute dating

Absolute dating methods are carried out in a laboratory. Absolute dates must agree with dates from other relative methods in order to be valid. The most widely used and accepted form of absolute dating is radioactive decay dating.

Radioactive decay dating. Radioactive decay refers to the process in which a radioactive form of an element is converted into a nonradioactive product at a regular rate. The nucleus of every radioactive element (such as radium and uranium) spontaneously disintegrates over time, transforming itself into the nucleus of an atom of a different element. In the process of disintegration, the atom gives off radiation (energy emitted in the form of waves). Hence the term radioactive decay. Each element decays at its own rate, unaffected by external physical conditions. By measuring the amount of original and transformed atoms in an object, scientists can determine the age of that object.

Words to Know

Dendrochronology: Also known as tree-ring dating, the science concerned with determining the age of trees by examining their growth rings.

Half-life: Measurement of the time it takes for one-half of a radioactive substance to decay.

Radioactive decay: The predictable manner in which a population of atoms of a radioactive element spontaneously disintegrate over time.

Stratigraphy: Study of layers of rocks or the objects embedded within those layers.

The age of the remains of plants, animals, and other organic material can be determined by measuring the amount of carbon-14 contained in that material. Carbon-14, a radioactive form of the element carbon, is created in the atmosphere by cosmic rays (invisible, high-energy particles that constantly bombard Earth from all directions in space). When carbon-14 falls to Earth, it is absorbed by plants. These plants are eaten by animals who, in turn, are eaten by even larger animals. Eventually, the entire ecosystem (community of plants and animals) of the planet, including humans, is filled with a concentration of carbon-14. As long as an organism is alive, the supply of carbon-14 is replenished. When the organism dies, the supply stops, and the carbon-14 contained in the organism begins to spontaneously decay into nitrogen-14. The time it takes for one-half of the carbon-14 to decay (a period called a half-life) is 5,730 years. By measuring the amount of carbon-14 remaining, scientists can pinpoint the exact date of the organism's death. The range of conventional radiocarbon dating is 30,000 to 40,000 years. With sensitive instrumentation, this range can be extended to 70,000 years.

In addition to the radiocarbon dating technique, scientists have developed other dating methods based on the transformation of one element into another. These include the uranium-thorium method, the potassium-argon method, and the rubidium-strontium method.

Thermoluminescence. Thermoluminescence (pronounced ther-moeloo-mi-NES-ence) dating is very useful for determining the age of pottery.

When a piece of pottery is heated in a laboratory at temperatures more than 930°F (500°C), electrons from quartz and other minerals in the pottery clay emit light. The older the pottery, the brighter the light that will be emitted. Using thermoluminescence, pottery pieces as old as 100,000 years can be dated with precision.

Tree-ring dating. Known as dendrochronology (pronounced den-dro-crow-NOL-o-gee), tree-ring dating is based on the fact that trees produce one growth ring each year. Narrow rings grow in cold or dry years, and wide rings grow in warm or wet years. The rings form a distinctive pattern, which is the same for all members in a given species and geographical area. Thus, the growth pattern of a tree of a known age can be used as a standard to determine the age of similar trees. The ages of buildings and archaeological sites can also be determined by examining the ring patterns of the trees used in their construction. Dendrochronology has a range of 1 to 10,000 years or more.

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dating methods

dating methods The methods used to determine the relative or absolute age of rocks, fossils, or remains of archaeological interest. A relative time scale, constructed in the last century, is based on correlations between palaeontological and stratigraphic data. The rate at which sediments accumulate can also be used for dating (see varve). Absolute dating relies on the decay of radioactive isotopes of elements present in the material to be dated (see decay constant; decay curve; decay series; isotopic dating; radiocarbon dating; and radiometric dating).

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